[Piglit] Cleanup code

[Dylan Baker]

This series accomplishes 3 basic things:
1) It removes all of the imports that are unused
2) It replaces implicit relative imports with explicit ones, either
using the relative import status "from .<modules> import ..." or by
using absolute imports
3) the last patch removes a couple of dead methods in core.py, these were
spotted by the vulture tool (a dead code detector for python), as
unused.